Get a VFC AEG. It is the best upgrade platform, stock performance and reliability are far above average.
GBBR isn't THAT complicated, just don't expect to be able to fire off a lot of rounds in succession without cooldown issues, don't expect to be able to play in the cold, and don't expect to have the same consistency as AEG.
Guardians of Asgaard - KF32
VFC H&K 416C
VFC KAC SR16 JL Billet SBR
TM FN Five-seveN